There are certain molded plastic products which by virtue of their design should be molded, shaped and finished in the mold cavity of a closed mold such as the cavities conventionally used in injection molding. However, because of their design, molding in such cavities is not feasible. Such parts are used in various types of products including furniture, architectural, appliance and autcmotive products. While the following description will focus on automotive products, it will be understood that such focus is only exemplary as is not to be considered as a limitation on the utility or scope of the invention.
One example of a product for which state of the art molding methods are inadequate is the body side molding applied by automobile manufacturers ro the side panels of automobiles to protect the panels and their finish from indentation and paint damage resulting from being hit such as by the open door of an adjacent vehicle. Another example is a similar rub strip applied to the bumpers of cars to provide an impact and abrasion resistant defense for the bumper and for the object contacted by the bumper. These rub strips are designed into the ornamental appearance of the vehicle and therefore must have a finished and decorative appearance. To satisfy this function they should be made as a unitary part with all visible surfaces having a finished and ornamental appearance. This can be accomplished by injection molding. However, injection molding has serious limitations.
In many designs these strips are long ribbon-like members and in the case of rub strips for bumpers may have a length of five, six or more feet. To injection mold such a product requires very large massive molds. Such molds, built to withstand the high pressures required for injection molding, must be of tool steel, have thick walls and are very expensive. Further, by virtue of the total mass of metal involved substantial quantities of thermal energy are required to heat them and substantial cooling capaciry may have to be provided to cool the injected charge. This requires expensive, large capacity support equipment and lengthens the molding cycle. All of this adds up to a cost factor which limits injection molding for such parts.
There are other deficiencies of injection molding when applied to such products. Some of the products require inserts such as a metallic reinforcement to be incorporated as an integral part of the product. The placement of the inserts into large multi-cavity injection molding dies requires holding them open for significantly longer periods, lengthening the molding cycle and increasing the operating costs.
A further limitation is that injection molding methods have only limited capacity to provide an acceptable product when the product requires a plastic having a high level of an individually visible filler such as metallic particles. Because of the flow patterns generated within the mold during high pressure injection, uniformity of filler distribution within the plastic is frequently not possible, giving the finished product an unacceptable appearance. Because of these limitations, these products have up to now been manufactured by extrusion. However, extrusion also has a number of disadvantages. One of the most important of these disadvantages is the fact it cannot produce a product with molded ends in a single operation. There is no method by which the ends of a product such as a body side molding or bumper rub strip can be shaped and finished as part of the initial extrusion step. To deal with this problem, one of the state of the art practices has been to extrude the main body of the part which is then placed in a mold and the ends molded on to it. This requires the cost of additional tools and the use of injection moding equipment. Thus, this method requires the cost burden of expensive equipment. In addition, it is a labor intensive procedure and, therefore, costly. Further it normally does not produce a product having the quality of appearance of a product made in a single step. Another state of the art practice has been to mold, as by injection, separate end pieces which are then bonded to the ends of the extruded strip. No matter how much care, labor and expense is invested in this procedure the resulting product the bond lines in the final product are all but impossible to conceal for various reasons including among others the difference in shrinkage between parts made under different pressures and molding under low pressure conditions coupled with the differences which occur within the daily practice of the processes. Furthermore the procedure also is labor intensive and, thus, expensive.
A serious limitation on extrusion is that in many cases it is not applicable where reinforcement members must be incorporated as an integral part of the final product. Where the reinforcement is a strand-like member such as continuous fiber glass filaments or one or more wires or a woven cable or a continuous ribbon such as of stainless steel or aluminium, this does not present a serious problem. However, if the insert is in the form of discrete pieces or its ends have to be recessed from the ends of the part, then the reinforcement must be assembled to and bonded to the plastic subsequent to the extrusion operation has been completed. If the strip has accessories on it such as grommets or fasteners or the like preassembled to it, the only way to use extrusion is to extrude the plastic first and subsequently bond it to the reinforcement. This is a slow, labor intensive and expensive procedure. Despite the shortcomings of the plastic molding technology for the manufacture of such parts as automotive rub strips that have been described in the preceding paragraphs, this technology was the state of the art until this invention.
Another limitation upon conventional molding techniques has been their impracticality when the material incorporates metallics. The high pressures and flow patterns characteristic of injection molding produce swirls and bleedouts in the plastic. These normally are not a problem when the plastic is an opaque, single color but becomes a serious problem when the plastic contains visual particulates such as metallics which make these patterns visible. While extrusion can successfully extrude parts with acceptable uniformity of metallics distribution, the need for finished ends becomes an insurmountable problem. Neither the post-extrusion molding technique nor the bonded-on end technique can hide the pattern interruption which is inherent where the original and subsequently molded parts are joined.